1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as employed in medicine for examination of patients. The present invention in particular concerns a method for improved interventional imaging in MRT using contrast agent liquids.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method for over 20 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal that is acquired by appropriate reception coils. By the use of inhomogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions, which is generally designated as “spatial coding”.
The acquisition of the data in MRT ensues in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by means of Fourier transformation. The spatial coding of the subject that spans k-space ensues by means of gradients in all three spatial directions. Differentiation is made between the slice selection (establishes an acquisition slice in the subject, typically the x-axis) and the phase coding (determines the second dimensional within the slice, typically the y-axis). Moreover, the selected slice can be subdivided into further slices by phase coding along the z-axis.
A slice is thus initially selectively excited, for example in the z-direction, and a phase coding is possibly implemented in the z-direction. The coding of the spatial information in the slice ensues via a combined phase and frequency coding by means of both of these aforementioned orthogonal gradient fields, which are generated by the likewise aforementioned gradient coils in the x- and y-directions in the example of a slice excited in the z-direction.
In order to acquire data from an entire slice of the subject to be examined, the imaging sequence (for example a gradient echo sequence, FLASH) is repeated N times for different values of the phase coding gradients, for instance Gy. The temporal separation of the respectively excited RF pulses is designated as a repetition time TR. The magnetic resonance signal (for example the gradient echo signal) is likewise sampled, digitized, and stored N times in every sequence pass via the Δt-clocked ADC (analog-digital converter) in equidistant time steps Δt in the presence of the read-out gradient Gx. In this manner, a number matrix created row-by-row (matrix in k-space, or k-matrix) with N×N data points is obtained. An MR image of the considered slice can be directly reconstructed with a resolution of N×N pixels from this data set via a Fourier transformation (a symmetric matrix with N×N points is only one example, asymmetrical matrices can be generated as well). For physical reasons, the values in the region of the center of the k-matrix primarily contain information about the contrast, the values in the boundary region of the k-matrix predominantly contain information regarding the resolution of the transformed MRT image. Slice images of the human body can be acquired in all directions in the manner just described. MRT as a slice image method in medical diagnostics has primarily excelled as a “non-invasive” examination method. Nevertheless, particularly in angiographic exposures (i.e. exposures of the blood vessels in the human body, especially in organs supplied (perfused) with blood), the contrasting in native MR imaging sets limits that can be significantly expanded by the use of contrast agents. The effectiveness of contrast agents in magnetic resonance tomography is generally based on an influence on the parameters significant to the contrast such as, for example, the longitudinal or transversal relaxation time T1 or T2. Trivalent gadolinium Gd3+ that has a T1-reducing effect is prevalent in clinical application. By bonding in complexes known as chelate complexes (DTPA, diethylenetriaminepentaacetic acid), gadolinium loses its toxicity such that Gd-DTPA can normally be applied intravenously. A vein is selected that leads directly to the heart, which ultimately distributes the contrast agent in the entire arterial system. In popular sequences (T1-weighted spin echo sequence, gradient echo sequence, etc.), the accelerated T1 relaxation time effects an increase of the MR signal, thus a brighter representation of the appertaining tissue in the MR image. Sharp and high-contrast images of, for example, head, throat, heart or kidney vessels can be achieved in this manner.
T1 time-reducing contrast agents normally represent low-molecular-weight contrast agents that remain in the vessel only a short time and then diffuse into the interstitial tissue. Alternatively, contrast agents known as “blood pool contrast agents” have been developed that remain in the blood vessels due to their size and that do not diffuse into the interstitial tissue (connective tissue) like low-molecular-weight magnetic resonance contrast agents.
If, in the framework of a vascular intervention, a catheter is introduced into vessels enriched with gadolinium-containing contrast agents, which vessels as such exhibit a high signal intensity, situations frequently occur (for example in the case of an embolization or a stent placement) in which the blood flow distal to the catheter is of great importance and must be monitored. In conventional angiography under x-ray radiography, the flow rates are checked via injection of x-ray contrast agent via the catheter. The vessel system distal to the catheter thereby contrasts and significant information regarding vessel openness, vessel wall condition, flow speed and flow characteristic is acquired. If it is desired to do this monitoring in the same manner under MRT, due to the high signal intensity (explained above) of the contrast agent-enriched blood given use of the typical highly-attenuated, paramagnetic, T1 time-shortening contrast agent such as GadDTPA (Magnevist®), one would attempt to increase the signal intensity. However, it is difficult to make the already-bright blood in the MRT image even brighter. A “contrast agent” ideal for such situations should therefore decrease the signal intensity.
According to the prior art, this ensues, for example, by injection of magnetic liquids that induce susceptibility artifacts and that significantly weaken or obliterate the magnetic resonance signal. The use of unattenuated or slightly attenuated gadolinium-DTPA-containing contrast agents or of iron oxide contrast agents (for example SPIO, USPIO) is typical. This method is limited, however, to only a few injection measurements because the recommended highest dose of these substances is very low.
As an alternative, it is possible to inject a proton-poor or proton-free substance that suppresses the blood for a short time. The injection of CO2 is cited as an example for this, which was proposed on the basis of animal tests by Wacker et al (Wacker et al, MR Imaging-Guided Vascular Procedures using CO2 as a contrast agent, AM Journal Roentgenological 2003; 181: 485-489) and which is described in United States Patent Application Publication No. 2004/0039278. CO2 is in fact a suitable contrast agent that has already been used for a long time in interventional radiology; nevertheless, many users (doctors) balk at injecting a gas into blood vessels. A disadvantage is also that this method can be applied only below the diaphragm. The application of CO2 at the heart or at cerebral vessels is forbidden due to the embolism risk that exists. An alternative solution is of great interest, particularly with regard to both of these cited vessel systems that are important for vascular interventions.
An alternative to direct injection of a contrast agent, for example, would be to saturate or to invert the blood signal itself outside of the image plane. Disadvantages of this method are the short T1 time of the blood (approximately 1500 ms, approximately 100 ms after contrast agent administration) as well as the limitation to applications with sufficient blood flow.